This invention relates to a scintillation-type x-ray detector and to radiology apparatus employing such detectors. In certain kinds of present-day radiology apparatus, the direct imprinting of a photographic film is replaced by detection of the quantity of x-rays received at points, this detection resulting in an electrical signal which is characteristic of each point. These signals are then processed by computer to reconstitute an image.
This procedure requires that the x-ray beam be scanned and its intensity be measured at instants, which consequently necessitates mechanical equipment which becomes larger as the size of the moving parts such as the detector become larger.
One object of the invention is to provide a detector of small bulk which is nevertheless of high sensitivity.
There exist a number of kinds of x-ray detectors which employ either an ionization chamber, or photo-conductive cells, or scintillation crystals.
The detector according to the invention employs a scintillator crystal which emits light in response to x-ray photons, which light is transmitted to a photomultiplier via a light collector.
The x-ray photons pass through the crystal and some of them are absorbed by the crystal. At the point at which it is absorbed by the crystal, each x-ray photon which is absorbed produces a number of light photons which are emitted in all directions. These, before they reach the surface, follow a path within the crystal whose length varies according to the direction in which they are emitted. Only a portion of the photons which strike a surface emerge, the others being reflected into the interior of the crystal where they follow one or more other paths. This portion is a function in particular of the respective refractive indices of the crystal and the medium adjacent to it, and of the state of the said surface.
The material used for the scintillator crystal thus needs to exhibit high absorption for the x-ray photons to be detected, a high quantum yield, low optical absorption for the emitted photons, and short persistence since the variations over time are considerable. Materials which are particularly suitable are calcium tungstate (CaWO.sub.4), bismuth germanate [Bi.sub.4 (G.sub.e O.sub.4).sub.3 ] and oxygen-doped zinc telluride (Zn Te : O).
The shape of the scintillating crystal is dictated by the following considerations: The entry area for the x-ray photons corresponds to the cross section of the x-ray beam to be detected. The crystal therefore has an entry area which is slightly larger in size than the x-ray beam, to avoid any lining-up difficulties, and only its thickness has to be optimized. The thickness is selected in such a way that the number of light photons which emerge from the crystal is a maximum for a given number of incident x-ray photons, taking into account, as explained above, the absorption of the x-rays, which should be a maximum, the absorption of the light photons, which should be a minimum, and the portion of light photons which are reflected from the faces into the interior of the crystal instead of emerging.
The shape of the crystal having been established in this way, it is necessary that the maximum number of light photons emerging from the crystal should pass through the window of the photomultiplier instead of being reflected when reaching the crystel/photomultiplier interface. The latter is generally coupled to the crystal by a coupler or collector. The function of the latter is to conduct light from the crystal to the photomultiplier while allowing for the difference in size between the two. In addition, so that the minimum amount of light shall be reflected at the interfaces, the refractive index of the material forming the coupler is selected to be as close as possible to the square root of the product of the index of the crystal multiplied by the index of the entry window of the photomultiplier.
The entry face of the crystal is covered with a thin reflective layer so that light rays which attempt to emerge from this face are thus reflected toward the opposite face through the crystal, the layer in question being permeable to x-ray photons.
These arrangements give rise to a certain number of disadvantages in the case of radiology apparatus having movable detectors.
Firstly, they do not enable the radiation emitted by the side-faces of the crystal to be reflected to the photomultiplier. The radiation which is lost in this way causes a corresponding reduction in the sensitivity of the detector since not all the light photons produced by the absorption of the x-ray photons are measured. This is particularly awkward in the case of radiological apparatus equipped with a number of detectors which are small but nevertheless need to be of high sensitivity.
To overcome this disadvantage, the light photons emitted by at least a portion of the side faces of the crystal are diverted back to the photomultiplier by reflection at reflective surfaces. The position and shape of these surfaces have to be such that they are unable to divert light onto the crystal whatever the direction of the rays likely to strike them.
The detector according to the invention, which is formed by a scintillator crystal which receives a beam of x-rays from a source and which transmits the light rays which are emitted in response to the x-rays which pass through it to a photomultiplier, is chiefly characterized by the fact that at least a portion of the light radiation which is emitted by the crystal and which does not impinge on the photomultiplier directly, is diverted toward the latter by reflection at a reflective surface whose intersection with a plane containing the x-ray beam and the photomultiplier is a curve whose evolute is external or tangent to the outline of the crystal in the said plane.
Another disadvantage is the space taken up by the photomultiplier in the direction of the x-ray beam, which makes it impossible to juxtapose the scintillator crystals of a plurality of detectors positioned side-by-side.
To remedy this, the photomultiplier is arranged obliquely to the direction of the x-ray beam which impinges on the crystal, at an angle which may vary from one detector to the next. The reflective surfaces are arranged in such a way that the photons emerging from those faces of the crystal which are not situated facing the photomultiplier are diverted toward the latter.
In accordance with another feature of the invention the crystal, which has a plurality of faces, is coupled to the photomultiplier by a collector which is formed from a transparent material which receives the light radiation emitted by the crystal, the said collector being covered with a reflective layer which diverts light back into the interior, the said photomultiplier being situated in a direction which is oblique with respect to the direction of the beam, the outline of the reflective surfaces in the plane containing the x-ray beam and the photomultiplier being a curve made up of a plurality of segments each corresponding to one face of the crystal and each connecting up with the neighboring segment, the reflective surfaces defined by each segment receiving both the light emitted by the crystal and the light reflected by the surfaces defined by adjoining segments, and reflecting the light in the direction of the photomultiplier and in the direction of the surfaces defined by adjoining segments.
Other features will become apparent in the course of the description of particular embodiments which is given below, and from the appended drawings.